Mitigation of photodamage in analytical reactions

ABSTRACT

Compositions, devices, systems and methods for reducing and/or preventing photodamage of one or more reactants in illuminated analytical reactions by one or more of incorporating photodamage mitigating agents within the reaction mixture and/or interrogating different observation regions of the reaction mixture for a period that is less than a photodamage threshold period.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation U.S. patent application Ser. No.11/293,040 filed Dec. 2, 2005, the full disclosure of which isincorporated herein in its entirety for all purposes.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

A portion of this invention was made with government funding under NHGR1Grant No. 1 R01 HG003710-01, and the government has certain rights inthe invention.

BACKGROUND OF THE INVENTION

The use of optically detectable labeling groups, and particularly thosegroups having high quantum yields, e.g., fluorescent or chemiluminescentgroups, is ubiquitous throughout the fields of analytical chemistry,biochemistry and biology. In particular, by providing a highly visiblesignal associated with a given reaction, one can better monitor thatreaction as well as any potential effectors of that reaction. Suchanalyses are the basic tools of life science research in genomics,diagnostics, pharmaceutical research, and related fields.

To date, such analyses have generally been performed under conditionswhere the amounts of reactants are so far in excess that any adverseeffects of the optical event would be unnoticed. For example, suchanalyses based upon fluorescent labeling groups generally require theuse of an excitation radiation source directed at the reaction mixture,to excite the fluorescent labeling group, which is then separatelydetectable. However, prolonged exposure of chemical and biochemicalreactants to such light sources, alone, or when in the presence of othercomponents, e.g., the fluorescent groups, can lead, potentially, todamage to such reactants, e.g., proteins, enzymes, substrates, or thelike. As noted previously, however, the existing formats for suchreactions generally prevents any such effects from being problematic, oreven being noticed.

A variety of analytical techniques are being explored, however, thatdeviate from the previous formats, such that detrimental effects of suchphotodamage will have a more dramatic impact on the operation of thegiven analysis. In particular, real time analyses of reactions thatinclude fluorescent reagents can expose multiple different components tooptical energy. Additionally, reactions based upon increasingly smalleramounts of reagents, e.g., in microfluidic or nanofluidic reactionvessels or channels, or in “single molecule” analyses. As such, thepresent invention is directed at methods and compositions that preventor mitigate to some extent, the adverse effects of such photodamage, andalso to processes that benefit from such methods and/or compositions,among other useful processes and compositions.

BRIEF SUMMARY OF THE INVENTION

The present invention is generally directed to compositions, devices,systems and methods for reducing and/or eliminating photodamage and itseffects in illuminated reactions, and particularly those that utilizefluorescent and/or fluorogenic reactants.

In at least a first aspect, the invention provides a composition thatcomprises a first reactant, a second reactant, and a photodamagemitigating agent, where interaction of the first reactant with thesecond reactant under excitation illumination causes photodamage to thefirst reactant in the absence of the photodamage mitigating agent.

Relatedly, the invention further provides a composition that comprises aconfined enzyme, a substrate for said enzyme, wherein interaction of theenzyme with the substrate under excitation illumination causesphotodamage to the first reactant in the absence of a photodamagemitigating agent, and a photodamage mitigating agent.

The invention also provides devices that comprise a substrate having anobservation region, a first reactant immobilized within the observationregion, a second reactant disposed within the observation region,wherein interaction between the first and second reactants underexcitation illumination causes photodamage to the first reactant, and aphotodamage mitigating agent disposed within the observation region.

The invention also provides methods of performing an illuminatedreaction, comprising providing a substrate having a reaction mixturedisposed thereon, wherein the reaction mixture comprises a firstreactant, a second reactant and a photodamage mitigating agent, whereinthe photodamage mitigating agent reduces an amount of photodamage to thefirst reactant resulting from interaction of the first reactant with thesecond reactant under excitation illumination that would occur in theabsence of the photodamage mitigating agent. The reaction mixture isthen illuminated on the substrate, with an excitation illumination.

In additional aspects, the invention provides methods of performing anenzyme reaction, comprising providing an enzyme within a firstobservation region. The enzyme is contacted with a fluorescent orfluorogenic substrate for the enzyme, and an excitation radiation isdirected at and signals are detected from the first observation regionfor a period that is less than a photodamage threshold period.

Similarly, in other aspects, the invention provides methods ofmonitoring a base extension reaction, comprising providing a polymeraseenzyme within a first observation region, contacting the polymerase withat least a first fluorescent or fluorogenic nucleotide analog, andmonitoring a fluorescent signal emitted from the first observationregion in response to illumination with excitation radiation for aperiod that is less than a photodamage threshold period.

The invention also includes a system for analyzing an illuminatedreaction that is susceptible to photodamage when illuminated for aperiod longer than an photodamage threshold period. The system typicallycomprises a substrate having reagents for the reaction disposed thereon,a mounting stage supporting the substrate and configured to receive thesubstrate, an optical train positioned to be in optical communicationwith at least a portion of the substrate to illuminate the portion ofthe substrate and detect signals emanating therefrom, and a translationsystem operably coupled to the mounting stage or the optical train formoving one of the optical train and the substrate relative to the other.

Also provided are methods of performing an enzyme reaction, comprisingproviding an enzyme within an observation region, and contacting theenzyme with a fluorescent or fluorogenic substrate for the enzyme underexcitation illumination, in the presence of at least a first photodamagemitigating agent.

Other methods of the invention include methods of monitoring a baseextension reaction, comprising providing a polymerase enzyme within anobservation region, contacting the polymerase with at least a firstfluorescent or fluorogenic nucleotide analog in the presence of at leasta first photodamage mitigating agent, and monitoring a fluorescentsignal emitted from the observation region in response to illuminationwith excitation radiation.

Also included are methods of monitoring a reaction mixture comprising atleast a first enzyme and a fluorescent or fluorogenic substrate for thefirst enzyme, where the method comprises directing an excitationradiation at a first observation region for a first period that is lessthan a photodamage threshold period.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a proposed mechanism ofphotodamage to DNA polymerase in template dependent synthesis usingfluorescent nucleotide analogs while under excitation illumination.

FIG. 2A-2B are images of agarose gels of DNA synthesis products made inthe presence of fluorescent nucleotide analogs and under selectiveillumination with laser excitation light. Shown are products ofsynthesis reaction mixtures in the presence and absence of differentphotodamage mitigating agents.

FIG. 3A-3C are images of DNA synthesized on a planar substrate usingfluorescent nucleotide analogs while being selectively illuminated atthe excitation wavelengths of the fluorescent analogs. Shown aresubstrates subjected to the reaction mixtures in the presence andabsence of different photodamage mitigating agents.

FIG. 4A-4B are images of arrays of zero mode waveguides havingimmobilized DNA polymerase disposed in the waveguides, and applied intemplate directed synthesis of DNA using fluorescent nucleotide analogs,while being selectively illuminated with lasers at the fluorescentanalogs' excitation wavelengths.

FIG. 5 is a schematic illustration of a step and repeat analysis methodto avoid the impacts of excessive photodamage on assay substrates.

FIGS. 6A and 6B provide a schematic comparison of a non-overlapping stepand repeat interrogation and a scan mode interrogation.

FIG. 7 is a schematic illustration of a system for carrying out certainaspects of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is generally directed to methods of performingimproved illuminated reactions, and particularly reactions that employfluorescent or fluorogenic reactants, that mitigate the effects ofand/or reduce photodamage to the various reactants present in suchreactions. The invention includes methods for preventing or reducingsuch photodamage as well as methods for mitigating the impacts suchphotodamage might have on an overall analysis, as well as combinationsof these.

While the invention is generally applicable to any of a variety ofoptical assays that require substantial illumination and/orphotoactivated conversion or excitation of chemical groups, e.g.,fluorophores, it finds greatest utility in analyses that utilize verylimited concentrations of reactants that might be subject tophotodamage. As will be appreciated, in such reagent limited analyses,any degradation of a critical reagent will dramatically impact theanalysis, by further limiting the reagent.

One particularly apt example of analyses that benefit from the inventionare single molecule biological analyses, including, inter alia, singlemolecule nucleic acid sequencing analyses, single molecule enzymeanalyses, hybridization assays, e.g., antibody assays, nucleic acidhybridization assays, and the like, where the reagents of primary importare subjected to prolonged illumination with relatively concentratedlight sources, e.g., lasers or other concentrated light sources, i.e.,mercury, xenon, halogen or other lamps, in an environment wherephotoconversion/excitation is occurring, with its associated generationof products.

With reference to nucleic acid analyses, it has been observed that intemplate directed synthesis of nucleic acids using fluorescentnucleotide analogs as the substrate, that prolonged illumination undersuch conditions yields substantial degradation in the ability of thepolymerase to synthesize such DNA (See FIG. 3A, and Example 1). Damageor even inactivation of polymerase enzymes can seriously detract fromthe ability of the polymerase to process longer strands of nucleicacids. This reduction in processivity of the enzyme, in turn, leads to areduction in read lengths for sequencing processes that identifysequence constituents based upon their incorporation into the nascentstrand. As is appreciated in the art of genetic analysis, the length ofcontiguous reads of sequence directly impacts the ability to assemblegenomic information from segments of genomic DNA. A proposed mechanismfor this photodamage is shown in FIG. 1. As shown, a fluorophore excitedby exposure to electromagnetic radiation at an excitation wavelength cantransition into a triplet state. Subsequent relaxation of the tripletstate fluorophore can then lead to generation of reactive oxygenspecies, which can, in turn, damage one or both of the fluorophore orthe enzyme processing the fluorophore, e.g., the polymerase.Accordingly, oxygen scavengers and/or reducing agents are included toprevent the formation of reactive oxygen.

In general terms, the invention is generally directed to the performanceof illuminated reaction analyses, where such analyses are illuminatedfor an amount of time that still permits the effective performance ofthe analysis. In particularly preferred aspects, illuminated analysisrefers to an analytical reaction that is occurring while beingilluminated, e.g., with excitation radiation, so as to evaluate theproduction, consumption and/or conversion of luminescent, e.g.,fluorescent reactants and/or products As used herein, the amount of timean illuminated analysis may be carried out before photodamage sosubstantially impacts the reactants to render the analysis non-useful,is referred to as the photodamage threshold period. In terms of theinvention, the photodamage threshold period is preferably that period ofilluminated analysis during which such photodamage occurs so as toreduce the rate of the subject reaction by at least 20% over the samereaction in the absence of such illumination, more preferably, more than50%, and in some cases, more than 90%, e.g., causing a 90% reduction inthe reaction rate of the system, or a 90% reduction in the amount ofproduct produced during a given time frame. It is an object of theinvention to perform an illuminated analysis within the photodamagethreshold period. This is generally accomplished in alternative ways.First, performing a given reaction within the foregoing parameters andin accordance with the invention or aspects thereof, may includeperforming the reaction for a period of time that is less than thephotodamage threshold period. Second, the reaction may be configured toincrease the length of the photodamage threshold period, or third, itmay include a combination of these approaches.

As will be appreciated, the photodamage sought to be prevented by themethods and compositions of the invention is not merely photodamage tofluorescent reagents, e.g., photobleaching, but is instead directed toprevention or reduction of the downstream effects of such photodamage toother reagents that are of limited quantity in a reaction mixture, andas such, their limited presence is more greatly impacted by even slightlosses due to photodamage, and particularly reactive proteins orenzymes, which, without being bound to a theory of operation, mayinclude damage to the enzymes or reactive proteins or irreversibleinteractions between such enzymes or proteins and the photodamagedreagents. As suggested by the foregoing, photodamage generally refers toan alteration in a given reagent, reactant or the like, that causes suchreagent to have altered functionality in a desired reaction, e.g.,reduced activity, reduced specificity, or a reduced ability to be actedupon, converted, or modified, by another molecule, that results from,either directly or indirectly, a photo-induced reaction, e.g., aphoto-induced reaction creates a damaged reactant that interacts withand causes damage to one or more other reactants. Typically, suchphotoreaction directly impacts either the reactant of interest, e.g.,direct photodamage, or impacts a reactant within one, two or threereactive steps of such reactant of interest.

As generally referred to herein, such limited quantity reagents orreactants may be present in solution, but at very limitedconcentrations, e.g., less than 200 nM, in some cases less than 10 nMand in still other cases, less than 10 pM. In preferred aspects,however, such limited quantity reagents or reactants refer to reactantsthat are immobilized, or otherwise confined within a given area, so asto provide limited quantity of reagents in that given area, and incertain cases, provide small numbers of molecules of such reagentswithin that given area, e.g., from 1 to 1000 individual molecules,preferably between 1 and 10 molecules. As will be appreciated,photodamage of immobilized reactants in a given area will have asubstantial impact on the reactivity of that area, as other, non-damagedreactants are not free to diffuse into, and mask the damage effects.

While researchers have provided methods and compositions for limitingphotodamage to fluorophores, the negative impacts of downstreamphotodamage to enzymatic systems in the presence of and/or resultingfrom photodestruction of fluorescent reagents has not been readilyrecognized or addressed. For ease of discussion, the detrimental impactof the photodamage event, whether resulting from actual damage to agiven reagent or from interaction with a damaged reagent, is generallyreferred to herein as photodamage.

I. PREVENTION OF PHOTODAMAGE

In a first aspect, the invention is directed to methods and compositionsthat reduce the amount of photodamage that is done to one or morenon-fluorescent reactants during illumination, e.g., with an excitationradiation source. In particular, compositions are provided that yield areduction in the level of photodamage (or an increase in the photodamagethreshold period) as compared to such reactions in the absence of suchcompositions. As used herein, the components of such compositions thatprovide such effects are generally referred to as photodamage mitigatingagents. In particular, photodamage mitigating agents are provided in thecontext of the analytical reaction to reduce the level of photodamage(and/or increase the photodamage threshold period), that would otherwisehave occurred but for the presence of the photodamage mitigating agent.

Again, the definition of an agent as a photodamage mitigating agent isgenerally reflective of the impact that such agent has on the actualphotodamage event or the downstream impacts of that damage. As such, aphotodamage mitigating agent may prevent photodamage of one or morereagents, or it may mitigate the impact that a photodamaged reagent mayhave on a particular, limited reagent in the reaction of interest. Byway of example, an agent that blocks a detrimental interaction between aphotodamaged fluorescent compound and a critical enzyme component wouldstill be referred to as a photodamage mitigating agent, regardless ofthe fact that it did not prevent the initial photodamage to thefluorescent reagent.

Measurements of reduction of photodamage as a result of inclusion ortreatment with a photodamage mitigating agent may be characterized asproviding a reduction in the level of photodamage over an untreatedreaction. Further, characterization of a reduction in photodamagegenerally utilizes a comparison of reaction rates, e.g., enzymeactivity, and/or a comparison of the photodamage threshold period,between a treated reaction mixture and an untreated reaction mixture.

In the case of the present invention, the inclusion of photodamagemitigating agent(s) of the invention generally results in a reduction ofphotodamage of one or more reactants in a given reaction, as measured interms of prevented loss of reactivity, e.g., enzyme activity, in thesystem, of at least 10%, preferably, greater than 20%, and morepreferably, greater than about a 50% reduction, and in many casesgreater than a 90% and up to and greater than 99% reduction in suchphotodamage. By way of illustration, and purely for the purpose ofexample, when referring to reduction in photodamage as a measure ofenzyme activity in the presence and absence of the photodamagemitigating agent, if a reaction included a reaction mixture having 100units of enzyme activity that would, in the absence of a photodamagemitigating agent, and following illuminated analysis, yield a reactionmixture having only 50 units of activity, then a 10% reduction inphotodamage would yield a final reaction mixture of 55 units (e.g., 10%of the 50 units otherwise lost, would no longer be lost).

Without being bound to a particular theory or mechanism of operation, itis believed that at least one cause of photo-induced damage to enzymeactivity, particularly in the presence of fluorescence reagents, resultsfrom the direct interaction of the enzyme with photodamaged fluorescentreagents. Further, it is believed that this photodamage of thefluorescent reagents (and possibly additional damage to the enzyme) isat least partially mediated by reactive oxygen species that aregenerated during the relaxation of triplet state fluorophores in thepresence of molecular oxygen.

Accordingly, in at least a first aspect, the present invention isdirected to the inclusion within the illuminated reaction mixture of oneor more agents that function to block or otherwise minimize the pathwaysthat lead to such photodamage. Such agents include reducing agents oranti-fade agents that prevent the formation of the triplet statefluorophores (also referred to as triplet state quenchers), as well asoxygen scavenging agents, that remove oxygen and reactive oxygen speciesfrom the reaction mixture, thus preventing downstream damage to enzymeswithin the system.

A variety of reducing agents or anti-fade agents may be used as tripletstate quenchers, including, for example, ascorbic acid, dithiothreitol(DTT), mercaptoethylamine (MEA), β-mercaptoethanol (BME), n-propylgallate, p-phenylenediamene (PPD), hydroquinone, sodium azide (NaN₃),diazobicyclooctane (DABCO), as well as commercially available anti fadeagents, such as Fluoroguard (available from BioRad Laboratories, Inc.,Hercules, Calif.), Citifluor antifadants (Citifluor, Ltd., London, UK),ProLong, SlowFade, and SlowFade Light (Invitrogen/Molecular Probes,Eugene, Oreg.).

Likewise, a number of singlet oxygen quenchers may be used to eliminateor reduce reactive oxygen species, including, for example, enzymaticsystems, e.g., superoxide dimutase, glucose oxidase/catalase (GO/Cat),glutathione peroxidase, or combinations of these with other enzymes, orthiol based quenchers e.g. ergothioneine, methionine, cysteine,beta-dimethyl cysteine (penicillamine), mercaptopropionylglycine, MESNA,glutathione, dithiothreitol (as noted above for a reducing agent),N-acetyl cysteine and captopril (See, e.g., Biochem Soc. Trans. 1990December; 18(6): 1054-6). Also, biological singlet oxygen quenchers maybe employed such as lycopene, gamma-carotene, astazanthin,canthazanthin, alpha-carotene, beta-carotene, and its analogs (See,e.g., Carcinogenesis vol. 18 no. 1 pp. 89-92, 1997), bixin, zeaxanthin,lutein, bilirubin, biliverdin, and tocopherols (See, e.g., Biochem SocTrans. 1990 December; 18(6): 1054-6 ref.) as well as polyene dialdehydes(Carcinogenesis vol. 18 no. 1 pp. 89-92, 1997) melatonin, vitamins E(∝-tocopheryl succinate and its analogs) and B₆ (pyridoxine1 and itsderivatives). Other chemical oxygen scavengers are also available, e.g.,hydrazine (N₂H₄), sodium sulfite (Na₂SO₃), hydroxylamine, glutathione,and N-acetylcysteine, and the like. In addition to the foregoing, inmany cases, the amount of singlet oxygen quenchers or scavengers may bereduced or eliminated by physically excluding oxygen from the reactionof interest by, e.g., degassing reagents, perfusion with inert gases, orthe like.

In accordance with the present invention, photodamage mitigating agentsmay generally be provided as a component of the reaction mixture, eitherthrough addition as an additive, either liquid or solid, or throughpredisposition and/or immobilization of the photodamage mitigatingagents within the region where the reaction is taking place. By way ofexample, in cases where the reaction of interest is confined to aparticular region or location, it may be desirable to immobilize orotherwise localize the photodamage mitigating agents within or proximalto that region. Likewise, where photodamage mitigating agent comprisescooperatively functioning components, e.g., dual enzyme systems, it mayagain be desirable to localize such components relative to each other,as well as to the reaction of interest.

II. MITIGATION OF PHOTODAMAGE IMPACTS

In contrast and/or in addition to the use of photodamage mitigatingagents, the present invention also provides methods of mitigating theimpact of photodamage on the results of a given analytical operation byonly interrogating a reaction mixture, e.g., detecting fluorescentemission, during such portion of the illumination period before whichexcessive photodamage has occurred. This approach is particularly usefulin the optical interrogation of reactions where components of thereaction that are susceptible to photodamage are spatially confined onan assay plate or substrate, either through the presence of structuralconfinements and/or through immobilization of the components. Examplesof such confined reagents include surface immobilized or localizedreagents, e.g., surface immobilized or associated enzymes, antibodies,etc. that are interrogated upon the surface, e.g., through fluorescencescanning microscopy or scanning confocal microscopy, total internalreflectance microscopy or fluorometry, surface imaging, or the like.

As used herein, a substrate may comprise any of a variety of formats,from planar substrates, e.g., glass slides or planar surfaces within alarger structure, e.g., a multi-well plates such as 96 well, 384 welland 1536 well plates or regularly spaced micro- or nano-poroussubstrates, or such substrates may comprise more irregular porousmaterials, such as membranes, aerogels, fibrous mats, or the like, orthey may comprise particulate substrates, e.g., beads, spheres, metal orsemiconductor nanoparticles, or the like. In addition, for purposes ofdiscussion herein, whether a particular reagent is confined by virtue ofstructural barriers to its free movement, or is chemically tethered orimmobilized to a surface of a substrate, it will be described as being“confined”.

For example, in interrogating an enzyme reaction where such photodamagecan occur and where the enzyme is immobilized upon a substrate surface,prolonged exposure of a particular region will result in photodamage toor “burning in” of the enzyme immobilized within that region. In anumber of cases, a selected region of a substrate, including thereaction of interest will be interrogated. For purposes of discussion,such region is termed an “observation region.”

In accordance with the present invention, the “burn in” or at least theeffects of such burn in, are reduced or eliminated by illuminating andcollecting emission signals from a different observation region. Forease of discussion, the action of both illuminating and collectingemission signals from a reaction of interest, or a particularobservation region in which a reaction of interest is taking place, isreferred to as interrogating that reaction and/or that region. As willbe appreciated, interrogating a new observation region of a substratewill constitute newly illuminating a region and collecting emissionsignals from that newly illuminated region. Rephrased, as long as one isinterrogating a newly illuminated region, whether the burned region isstill being illuminated is not of major import, unless one is desirousof returning to interrogate that region at a later time.

In addition to the advantages of reducing photodamage, the process ofinterrogating different regions of a substrate over time also providesbenefits of being able to interrogate larger substrate areas with agiven light source than would have otherwise been possible withoutmodifying the nature of the illumination, e.g., expanding a laser spotsize by changing the illumination angle, e.g., to provide an elongatedlaser spot size (See, e.g., U.S. Pat. No. 6,881,312, incorporated hereinby reference in its entirety for all purposes), or passing theillumination through an optical train that alters the shape of theincident light spot on the substrate, e.g., providing a cylindrical lensto provide the illumination in a line format, or otherwise refocusingthe illumination to provide an expanded spot size or dimension.Notwithstanding the foregoing, it will be appreciated that the presentinvention is optionally combined with such optics that provide anexpanded illumination area, that is optionally used in addition toprocesses where such expanded illumination profile is then moved overthe substrate to interrogate different regions of the substrate overtime. FIG. 5 illustrates the movement of an interrogation spot regionover a substrate upon which a reaction of interest is being carried out,over time, to interrogate different regions of a substrate. As will beappreciated, used of a linear illumination spot over the substrate wouldmore rapidly illuminate larger areas of the substrate than the circularspot shown in FIG. 5. As shown in FIG. 5, the exemplary substratecomprises a plurality of arrays of smaller structural confinements (thatalso function as optical confinements in the form of zero modewaveguides), where each array or subset of arrays are included within aseparate structural confinement, e.g., a well in a multiwell substrateor plate. As will be appreciated, the interrogation function typicallyis carried out over a given region for a prolonged period of time thatis not longer than the photodamage threshold period. Typically, thiswill be for greater than 10 seconds, preferably greater than 1 minute,more preferably greater than 5 minutes, greater than 10 minutes, greaterthan 20 minutes, and in some cases, greater than 2 hours or greater than3 or more hours, but still less than the photodamage threshold period.

In addition to gaining additional interrogation area by moving theinterrogation region over the area of the substrate, the ability to movethat region, also provides an ability to adjust the mechanicalinterfaces with the substrate in a particular system or apparatus, so asto make regions available for interrogation that may have been otherwiseun-interrogatable in the particular system or apparatus. In particular,in a typical substrate analysis set-up, a substrate to be analyzed isfixed upon an analysis stage where portions of that substrate may beobscured from interrogation by the mounting structure of the analysisstage, e.g., clips, support structures, or the like. In accordance withcertain aspects of the invention, however, the movement of theinterrogation region provides the ability to alter, over time, theportions of the substrate that are obscured by the mounting structures.In a first example, rather than moving the optical train that providesillumination to a given region of the substrate, the substrate may bemoved relative to the interrogation optics. This may be accomplishedusing any of a variety of manipulation hardware or robotic set-ups. Forexample, a stepper/feeder apparatus is used that steps the substratethrough the interrogation zone of the optical train in a precisefashion. Such precise feeder apparatus' are well known in highperformance printing technologies, as well as in translational roboticsused in the semiconductor industry, e.g., in both analytical andmanufacturing applications. Such stepper feeders may include a roller orwheel assembly that contacts an upper surface of the overall substrate,and is rotated to provide motive force to the substrate in a precisefashion, to feed that substrate through the interrogation zone of theoptical train. In alternative aspects robotic systems may be used topick-up and re-orient a given substrate in order to interrogatedifferent regions of the substrate surface, or make a previouslyinaccessible region of the substrate accessible. Such robotic systemsare generally available from, e.g., Beckman, Inc., Tecan, Inc., CaliperLife Sciences, and the like.

In accordance with the invention, a reaction of interest within a firstobservation region is interrogated for a time period that is less than aphotodamage threshold period, as set forth elsewhere herein, and thenthe reaction of interest in a second, different observation region isinterrogated. In accordance with the present invention, the observationtypically includes confined reagents that are susceptible tophotodamage. As such, an observation region may include an area of aplanar or other substrate surface upon which are immobilized reagents,e.g., enzymes. Alternatively, the observation region may include aphysical confinement that constrains the reagents that are susceptibleto photodamage, including, e.g., microwells, nanowells, planar surfacesthat include hydrophobic barriers to confine reagents. As noted above,the present invention is particularly applicable to observation regionsin which the damage susceptible reagents are present at concentrationsor levels that photodamage greatly impacts the reaction progress. Thisis particularly the case in immobilized reaction systems whereadditional, excess amounts of reagents can not be provided in a bulksolution to obscure the impact of any damaged reagents.

The sequential interrogation of different observation regions maygenerally be repeated a large number of times, e.g., more than 10, morethan 100 more than 1000, or even more than 10,000 times, so long asobservation regions remain. The availability of multiple regions isgenerally limited only by the size of a discrete observation region,which may be defined by one or more of the nature and dimensions of anystructural confinements used, and the illumination spot size, and theoverall area of the analytical substrate. In general, this method ofstepping the interrogation region to another, preferably adjacentregion, and repeating the interrogation process is generally referred toas a “step and repeat” process.

Although described as a “step and repeat” method, in some embodimentswhere the interrogation region is moved across a substrate, thatmovement is not step-wise and iterative, but instead constitutes acontinuous motion, substantially continuous motion or a stepped movementor iterative motion whereby each iterative step interrogates a newregion that overlaps with some portion of the previously interrogatedregion or of the interrogation region across the substrate. Inparticular, a substrate may be moved continuously through aninterrogation zone of an optical system, whereby the interrogationregion moves continuously across the substrate being interrogated (in a“scan mode”). In accordance with preferred aspects, the speed ofmovement of the interrogation region is dictated by the amount of time agiven reaction zone, e.g., a structural or optical confinement, ZMW, orthe like, is desired to stay within the interrogation region, e.g., fora period less than the photodamage threshold period. FIG. 6A shows aschematic illustration of a non-overlapping step and repeatinterrogation method using a circular illumination spot. As shown, someportion of the substrate surface, indicated by hatching, is notsubjected to interrogation. In FIG. 6B, however, a scanning oroverlapping stepping process is used to interrogate larger portions ofthe surface area.

FIG. 7 is a schematic illustration of an overall system 700 useful forperforming the step and move operations on substrates in accordance withcertain aspects of the invention. As shown, a reaction substrate 702 isdisposed upon a translation stage 704. Stage 704 is typically coupled toappropriate robotics (schematically represented by armature 706) thatprovides lateral translation of the substrate 702, in two dimensions (xand y) over a fixed optical train 708. Although shown as being coupledto and rendering the translation of the substrate, it will beappreciated that alternative configurations could couple to translationsystem to the optical train to move that aspect of the system relativeto the substrate. Optical train 708 may comprise a variety of differentconfigurations useful for interrogating the substrate, includingappropriate excitation light sources, e.g., laser 718, focusing andfiltering optics, e.g., dichroic mirror 720, objective lens 710, imaginglens 712, prism 714, and detectors or detector arrays, e.g., detectorarray 716. One example of a particularly preferred optical train isdescribed in commonly owned U.S. patent application Ser. No. 11/201,768filed Aug. 11, 2005, and incorporated herein by reference in itsentirety for all purposes.

III. EXEMPLARY APPLICATIONS

As noted above, the methods and compositions of the invention are usefulin a broad range of optically detected analytical reactions, andparticularly those using photoluminescent or fluorescent reactants, andparticularly such reactions where the reagents that are susceptible tophotodamage are present at relatively low levels. One exemplaryapplication of the methods and compositions described herein is insingle molecule analytical reactions, where the reaction of a single, orvery limited number of molecules are observed in the analysis, such asobservation of the action of a single enzyme molecule. In particular,when an analysis is relying upon a small population of reagentmolecules, damage to any significant fraction of that population willhave a substantial impact on the analysis being performed.

One example of a single molecule analysis includes sequencing of nucleicacids by observing incorporation of nucleotides into a nascent nucleicacid sequence during template directed polymerase based synthesis. Suchmethods, generally referred to as “sequencing by incorporation,” involvethe observation of the addition of nucleotides or nucleotide analogs ina template dependent fashion in order to determine the sequence of thetemplate strand. A number of processes for performing this detectioninclude the use of fluorescently labeled nucleotide analogs within aconfined observation region, e.g., within a nanoscale well or tethered,either directly or indirectly to a surface. By illuminating anddetecting the fluorescent bases that are incorporated, or are to beincorporated into the nascent strand, one can ascertain the nature ofthe base, and as a result, the complementary base in the templatestrand.

One particularly preferred aspect of the invention is in conjunctionwith the sequencing by incorporation of nucleic acids within an opticalconfinement, such as a zero mode waveguide, in which one is observing anextremely small reaction volume in which one or only a few polymeraseenzymes and their fluorescent substrates may be present. Zero modewaveguides, and their use in sequencing applications is generallydescribed in U.S. Pat. No. 6,917,726, and preferred methods ofsequencing by incorporation are generally described in Published U.S.Patent Application No. 2003-0044781, the full disclosures of which areincorporated herein by reference in their entirety for all purposes.

As will be appreciated, prolonged interrogation of a limited populationof reagents, e.g., fluorescent analogs and confined polymerase enzymescan lead to photodamage of the various reagents to the point ofsubstantially impacting the activity or functionality of the polymeraseenzyme. In particular, it has been shown that prolonged illumination ofDNA polymerases involved in synthesis using fluorescent nucleotideanalogs results in a dramatic decrease in the enzyme's ability tosynthesize DNA. Without being bound to any theory of operation, it isbelieved that the photodamage event affects the catalytic region of theenzyme thus affecting either the ability of the enzyme to remaincomplexed with the template, or its ability to process additionalsynthesis.

In accordance with the present invention, the above-described sequencingreaction may be carried out in the presence of one or more photodamagemitigating agents, as described above. In preferred aspects, thesequencing reactions may be carried out in the presence of both areducing agent, such as DTT, MEA or BME, and an oxygen scavenger, suchas GO-Cat.

In general, the photodamage mitigating agents are present in thereaction mixture at levels sufficient to provide beneficial impact,e.g., reduced photodamage and/or extension of the photodamage thresholdperiod, but are not present at such levels as to interfere with thereaction of interest, e.g., the sequencing reaction. Concentrations ofthe components of a photodamage mitigating agent will generally vary byapplication. By way of example, reducing agents, such as DTT, MEA orBME, may generally be present at amounts of between about 100 μM and 500mM, and preferably between about 1 mM and about 200 mM, e.g., in somecases about 5 mM for DTT and 100 mM for MEA, but may vary from theseconcentrations. In the case of DTT, preferred concentrations range fromabout 1 mM to about 10 mM, while preferred ranges for MEA may be fromabout 10 mM to about 200 mM. Likewise, the concentration of oxygenscavengers will generally vary depending upon the application, the levelof oxygen present, the susceptibility of the system to reactive oxygenspecies, etc. For example, in sequencing reactions, oxygen scavengingenzyme systems, e.g., GO-Cat, are generally present at levels thatprovide effective oxygen scavenging without excessively impairing thedesired reactions, e.g., polymerase activity. Typically, this includesconcentrations of GO-Cat reagents within the reaction mixture that areanywhere from, e.g., up to about 5 μM Glucose Oxidase and up to about575 nM catalase, or 3 to 4 times typical GO-Cat concentrations, down to13 nM Glucose Oxidase and 1.5 nM catalase) or 0.01×GO-Catconcentrations. Typically, the concentrations will be between about0.01× to about 0.5× of typical GO-Cat concentrations as set forth above,and more preferably including or between about 0.1× and 0.25×GO-Cat. Forimmobilized oxygen mitigation systems, the amount of immobilizedreagents will generally provide activity levels that correspond to theactivity levels of the aforementioned concentrations in non-immobilizedformats. Precise amounts of reagents will generally depend upon therelative efficiency of the immobilization process, and resultingactivity of the immobilized components.

The following non-limiting examples are provided to further illuminatethe invention.

IV. EXAMPLES

Because of the value of single molecule analysis in nucleic acidsequencing applications, DNA polymerase systems were used to identifythe impact of photodamage and its solutions in accordance with thepresent invention. Initial assays were run in three differentconfigurations to identify the scope and/or nature of photodamage topolymerase reactions. These included a bulk DNA synthesis experiment, aflat surface based nucleic acid synthesis reaction, and synthesis withinan array of zero mode waveguides.

Example 1 Photodamage and Mitigation in Bulk Reaction Volumes

In a first assay, synthesis reaction mixtures contained a modified (φ29DNA polymerase, 300 nM DNA template, three native nucleosidetriphosphates (at 10 μM each) and a fluorescent dye labeled nucleosidepolyphosphate (at 10 μM) in synthesis buffer (50 mM Tris-HCl, pH 7.5, 75mM KCL, 20 mM (NH₄)₂SO₄, 10 mM BME, 0.7 mM MnCl₂). Each of the reactionswere carried out at room temperature (22° C.) for the desiredillumination period, ranging from 1 minute to one hour.

The experiment included two sets each of three different reactionmixtures: (1) a synthesis reaction using only native, e.g., unlabelednucleoside triphosphates; (2) a synthesis reaction including two nativenucleoside triphosphates, an Alexa 488 labeled dCTP analog, and an Alexa568 labeled dTTP; and (3) a synthesis reaction including two nativenucleoside triphosphates, an Alexa 488 labeled dC4P analog(tetraphosphate), and an Alexa 568 labeled dT4P. Each differentsynthesis reaction conditions included either no illumination or laserillumination during synthesis for five minutes with wavelengths of 488,568 and 647 nm, followed by 60 minutes of nonilluminated synthesis.

Following synthesis, the reaction products were separated on a 0.7%agarose gel under standard conditions. FIG. 2A provides an image of theSybr® Gold stained gel. As shown, lane 1 on the left, includes amolecular weight standard. The next two lanes (3 and 4, lane 2 isempty)) include the synthesis reaction including only unlabellednucleoside triphosphates (reaction conditions 1, above), in the absenceof laser illumination (−) and with laser illumination (+). Moving to thenext two lanes to the right (5 and 6) include similar reactions, butincluding labeled nucleoside triphosphates (reaction condition 2,above), while the right most lanes (7 and 8) include the labelednucleoside tetraphosphate analogs in the synthesis reaction (reactioncondition 3, above) (For a discussion of phosphate labeled nucleosidepolyphosphates, see, e.g., U.S. Pat. No. 6,399,335, and published U.S.Patent Application No. 2003/0124576, the full disclosures of which areincorporated herein by reference for all purposes).

As can be seen from the gel, a large amount of relatively high molecularweight DNA has been synthesized in the native reaction, both with andwithout laser illumination. In each of the cases utilizing labeledanalogs, the amount and relative size of the synthesized DNA is lessthan native conditions. Of particular note, however, is that in each ofthese latter two reactions, the laser illumination results in asubstantial decrease in the amount of higher molecular weight DNAproduced. Of further note, despite that reaction conditions areidentical for reaction conditions 2 and 3, except for the use oftetraphosphate analogs, the amount of lost DNA synthesis in theilluminated sample is proportionately greater in the labeledtriphosphate reaction. This is indicated by the ratio of DNA in theIlluminated sample to the nonilluminated sample for each reactioncondition (as determined by image scanning). In particular, the ratioDNA quantity in the gel lane of illuminated to nonilluminated in thenative reaction conditions is approximately 1 (1.10). When the reactionincludes labeled triphosphate analogs, this ratio drops to 0.27, whilethe use of tetraphosphate analogs drops this ratio to 0.56. These dataare suggestive that the photodamage effects may be caused by proximityor length of retention time of the fluorophor to the active site of theenzyme during illumination. This interpretation was strengthened bysimilar experiments performed with unlabeled nucleoside triphosphates,that were spiked with free dyes, e.g., not coupled to the analog, thatshowed little or no impact of illumination on synthesis.

Similarly, synthesis reactions using fluorescent analogs that wereilluminated at a nonexciting wavelength showed little or no impact onpolymerase activity, again, indicating that the excited and/orfluorescing analog mediated the damage to the polymerase activity insome measure. The various above-described experiments indicated thatphotodamage was greatest in the reactions that included the Alexa568 dyelabeled nucleotide analogs, further bolstering the suggestedphotophysical effect, as the Alexa568 dye is reported to be lessphotostable than the Alexa 488 dye. Additional experiments usingnon-incorporatable dye labeled analogs, e.g., not complementary to anybase in the template, provided little or no measurable photodamage. Allof the foregoing provides further apparent indication that the impact onpolymerase activity results from the presence of an excited dye labelednucleotide (or nucleotide analog) within the active site of thepolymerase enzyme, indicating some damage to the enzyme or irreversibleinteraction at the active site.

The experiments using dye labeled tetraphosphates (reaction condition 3,above) were repeated using three different mitigation treatments: (1) 10mM βME (standard conditions or negative control); (2) 5 mM DTT; and (3)100 mM MEA, with and without laser illumination as described above.Again, the synthesis products were separated on an agarose gel, an imageof which is shown in FIG. 2B. The gel was subjected to image scanning(Molecular Dynamics Typhoon 9400, with Typhoon scanner Control Version2; gel image quantified with Molecular Dynamiics ImageQuant ver. 5.2).The results of this analysis showed that in the absence of any changefrom standard conditions, e.g., including only 10 mM βME, the ratio ofproduct when exposed to laser illumination to that in the absence ofsuch illumination was 0.24. When DTT was added to the reaction mixture,the ratio improved to 0.59, while addition of MEA appeared to providecomplete or substantially complete protection (a ratio of 1.04) againstphoto-induced damage from a 5 minute illumination. These datademonstrate that the use of reducing agents as photodamage mitigatingagents appear to prevent loss of polymerase activity that occurs duringsynthesis that is occurring under laser excitation illumination.

Example 2 Photodamage and Mitigation in Surface Immobilized EnzymeSystems

Next, a GST-tagged φ29 polymerase was coated on the surface of a fusedsilica microscope slide, by depositing the polymerase over the slide andincubating the surface for 15 minutes on ice. Template dependentsynthesis of DNA was carried out on the surface using native nucleotidesand 10 μM Alexa488-labeled-dC4P and Alexa568-labeled-dT4P, whileilluminating a small semi-circular shaped laser spot on the slide. Theonly reducing agent present in the mixture was 10 mM βME. The slideswere exposed to laser illumination at 488 nm (1.1 mW) and 568 nm (1.8mW) with different positions being illuminated for 1 minute and for 5minutes. Following illumination, synthesis was allowed to continue for60 minutes using only native nucleotides. The slides were stained forthe presence of synthesized DNA using Sybr-gold. Images of theilluminated slides after 1 minute and 5 minutes are shown in FIG. 3A. Ascan be seen, the semicircular illumination region is devoid of anysynthesized DNA after only 1 minute of illumination, and the impact isshown to be greater after 5 minutes of illumination.

As synthesis was lacking even when non-illuminated synthesis was allowedto proceed for 60 minutes, it is indicative not only of photodamage topolymerase activity, but also that such damage is apparently lasting oreven permanent.

A similar experiment was carried out in the presence of differentmixtures of photodamage mitigating agents or concentrations thereof. Inparticular, as with Example 1, above, three different reaction mixtureswere used that included different mitigation treatments: (1) 10 mM βME(standard conditions or negative control) (same as shown in FIG. 3A);(2) 100 mM MEA; and (3) 100 mM MEA, 5 mM DTT and 1×GO Cat (1.3 μMGlucose Oxidase and 150 nM catalase). The results are shown in FIG. 3B.As can be seen, the reactions that included MEA showed a dramaticdecrease in the burned in image indicative of damaged polymeraseactivity, in both 1 minute and 5 minute illumination experiments. Theaddition of DTT and GO-Cat further reduced the level of damage topolymerase activity to the point that it was not discernible in the 1minute exposure, and was barely discernible after 5 minutes exposure.

While the presence of GO-Cat provides a substantial elimination ofphotodamage, the presence of relatively high concentrations of theseproteins may have adverse effects on certain applications, e.g., wheresuch reactions are based on relatively low levels of reactants, as suchprotein can mask, block or otherwise inhibit reactions of interest. Assuch, an additional experiment was carried out to determine effectivereduced levels for the various photodamage mitigating agents on apretreated surface. Briefly, a surface treated to provide selectivepolymerase immobilization was prepared and used for the titrationexperiments on the concentrations of GO-Cat. The experimental set up isset forth below.

A. Surface Preparation

Neutravidin was diluted at 1 mg/ml to 0.2 mg/ml in a solution of 1×BFA(0.05% Tween20, 150 mM KCl, 25 mM Tris-HCl pH 7.5, 5 mM DTT). Biotin-GSTtagged (φ29 polymerase was diluted to a concentration of approximately128 nM in 1×BFA, and equal volumes of the neutravidin solution andpolymerase solution were combined and incubated at 23° C. for 30minutes.

The neutravidin-polymerase mixture was then placed onto a gasketed fusedsilica slide having a PEG24-Biotin modified surface, and covered with acover slip. The slide was then incubated for 1 hour at 23° C. Followingincubation, the slide was washed 3 times in 1×BFA.

B. Synthesis/Illumination Experiments

The GO-Cat reagents were used to dilute with 2×MM reagent (2×prb-BME(100 mM Tris-HCl pH 7.5, 40 mM ammonium sulfate, 150 mM KCl), 200 mMMEA, 10 mM DTT, 0.4% glucose, 1.4 mM MnCl₂, 300 nM CL31 circulartemplate, 20 μM A488 dC4P, 20 μM A568 dT4P, 20 μM dATP and dGTP), 1:1 toyield final reaction mixtures having 100 mM MEA, 5 mM DTT, and 0, 0.02×,0.05× and 0.1×GO-CAT reagent. These diluted synthesis reagents (diluted2×MM with and without GO-CAT) were then deposited onto the gasketedslide, which was then illuminated at a suitable location with laserspots at 488 nm and 568 nm, for 5 minutes. Following laser illumination,the reaction mixture was replaced with the 1×postMM reagent (1×prb-BME(50 mM Tris-HCl pH 7.5, 20 mM ammonium sulfate, 75 mM KCl), 100 nM CL31circular template, 0.7 mM MnCl2, 2 μM A488-dUTP, 8 μM dTTP, 10 μM dATP,dCTP and dGTP) and incubated at 23° C. for 60 minutes withoutillumination. The resulting slide was washed twice with 1×BFA andstained with Sybr® Gold intercalating dye, and imaged. The resultingimages are shown in FIG. 3C. As can be seen, use of exceedingly lowlevels of GO-Cat provides beneficial impact on polymerase activity.However, the presence of the GO-Cat reagents at approximately 0.1× thestandard concentration provides nearly complete elimination ofpolymerase activity damage. The image data was then plotted (FIG. 3D) asa function of background normalized crater intensity volume vs. GO-Catconcentration. Again, suitable protection appears to be achieved at therelatively low added protein level of 0.1×GO-Cat.

Example 3 Photodamage and Mitigation on Nanostructured Reactive Surfaces

A similar set of experiments to those described above were performedusing DNA polymerase immobilized within zero mode waveguides in an arrayof waveguides. As above, the first experiment was designed to identifywhether laser illumination caused damage to immobilized polymeraseenzymes on nanostructured surfaces. The surfaces included ZMW arrays inwhich the polymerase enzyme was adsorbed to the surface. DNA synthesisusing dye labeled nucleoside tetraphosphates (Alexa488dC4P andAlexa568dT4P) was carried out with and without laser illumination (at488 and 568 nm) and the resulting product was again stained with Sybr®Gold. Images of the arrays are shown in FIG. 4A. The illuminationprofile is shown in the first panel (far left), while the image of thestained DNA product in the illuminated synthesis is shown in theadjacent panel (middle left). As can be seen, a negative image isapparent in the illuminated region corresponding to the illuminationpattern. The middle right and far right panels show non-illuminatedwaveguide arrays, and indicate substantially more DNA is present than inthe illuminated sample, again showing photo-induced damage to thepolymerase activity in the waveguide arrays.

FIG. 4B illustrates a first set of waveguide arrays in which similarsynthesis reactions were carried out in the presence of no additionalmitigation agents (e.g., only 10 mM βME), or in the presence of 5 mM DTTand 100 mM MEA. As can be seen addition of the DTT and MEA providessubstantial protection against damage to polymerase activity caused bylaser illumination, and appears to give reactions that producesubstantially equivalent amounts of DNA product as the non-illuminatedarrays. Additional experiments also showed improvements in the amount ofdamaged polymerase activity in the presence of 160 mM DTT, without MEA,although not as pronounced as in the presence of 5 mM DTT and 100 mMMEA.

Although described in some detail for purposes of illustration, it willbe readily appreciated that a number of variations known or appreciatedby those of skill in the art may be practiced within the scope ofpresent invention. Unless otherwise clear from the context or expresslystated, any concentration values provided herein are generally given interms of admixture values or percentages without regard to anyconversion that occurs upon or following addition of the particularcomponent of the mixture. To the extent not already expresslyincorporated herein, all published references and patent documentsreferred to in this disclosure are incorporated herein by reference intheir entirety for all purposes.

1-33. (canceled)
 34. A system for analyzing an illuminated reaction thatis susceptible to photodamage when illuminated for a period longer thanan photodamage threshold period, comprising: a substrate having reagentsfor the reaction disposed thereon, the reagents comprising a firstreactant, a second reactant and a photodamage mitigating agent, whereinthe photodamage mitigating agent reduces an amount of photodamage to thefirst reactant resulting from interaction of the first reactant with thesecond reactant under excitation illumination that would occur in theabsence of the photodamage mitigating agent; a mounting stage supportingthe substrate and configured to receive the substrate; an optical trainpositioned to be in optical communication with at least a portion of thesubstrate to illuminate the portion of the substrate and detect signalsemanating therefrom; a translation system operably coupled to themounting stage or the optical train for moving one of the optical trainand the substrate relative to the other. 35-38. (canceled)
 39. Thesystem of claim 34, wherein the first reactant comprises a nucleic acidpolymerase, and the second reactant comprises at least a firstfluorescently labeled nucleotide or nucleotide analog.
 40. The system ofclaim 34, wherein the photodamage mitigating agent is selected from anoxygen scavenging reagent and a triplet state quencher.
 41. The systemof claim 34, wherein the photodamage mitigating agent is selected fromsuperoxide dismutase, glucose oxidase/catalase (GO/Cat), glutathioneperoxidase, ergothioneine, methionine, cysteine, beta-dimethyl cysteine,mercaptopropionylglycine, MESNA, glutathione, N-acetyl cysteine,captopril, lycopene, gamma-carotene, astazanthin, canthazanthin,alpha-carotene, beta-carotene, bixin, zeaxanthin, lutein, bilirubin,biliverdin, tocopherols, polyene dialdehydes, melatonin, α-tocopherylsuccinate and its analogs, and pyridoxine1 and its derivatives,hydrazine (N₂H₄), sodium sulfite (Na₂SO₃), hydroxylamine, ascorbic acid,dithiothreitol (DTT), mercaptoethylamine (MEA), β-mercaptoethanol (BME),n-propyl gallate, p-phenylenediamene (PPD), hydroquinone, sodium azide(NaN₃), diazobicyclooctane (DABCO).
 42. The system of claim 34, whereinthe substrate comprises a plurality of discrete reaction regions eachhaving reagents for the reaction disposed therein.
 43. The system ofclaim 42, wherein the plurality of discrete reaction regions compriseconfined reaction wells.
 44. The system of claim 42, wherein theplurality of discrete reaction regions comprise a plurality of zero modewaveguides disposed on the substrate.
 45. The system of claim 34,wherein the translation system is programmed to move one of the opticaltrain or substrate relative to the other, to illuminate a second portionof the substrate, such that the first portion of the substrate isilluminated for less than the photodamage threshold period.
 46. A systemfor analyzing an illuminated reaction that is susceptible to photodamagewhen illuminated for a period longer than an photodamage thresholdperiod, comprising: a substrate having reagents for the reactiondisposed thereon; a mounting stage supporting the substrate andconfigured to receive the substrate; an optical train positioned to bein optical communication with at least a first portion of the substrateto illuminate the first portion of the substrate and detect signalsemanating therefrom; a translation system operably coupled to themounting stage or the optical train for moving one of the optical trainand the substrate relative to the other, wherein the translation systemis programmed to move one of the optical train or substrate relative tothe other, to illuminate a second portion of the substrate, such thatthe first portion of the substrate is illuminated for less than thephotodamage threshold period.
 47. The system of claim 46, wherein thetranslation system is programmed to move one of the optical train orsubstrate relative to the other, to illuminate a third portion of thesubstrate, such that the second portion of the substrate is illuminatedfor less than the photodamage threshold period.
 48. The system of claim47, wherein each of the first, second and third portions of thesubstrate comprises at least one discrete reaction region.
 49. Thesystem of claim 48, wherein each of the first, second and third portionsof the substrate comprises a plurality of discrete reaction regions. 50.The system of claim 46, wherein the reagents for the reaction comprise anucleic acid polymerase and at least a first fluorescently labelednucleotide or nucleotide analog.